Negligence per se vs. res ipsa loquitur: two shortcuts to proving carelessness

Negligence per se and res ipsa loquitur are two shortcuts to proving carelessness. The difference is in what the injured person relies on. With negligence per se, the person shows the defendant broke a safety law, and that violation stands in for proof of careless conduct. With res ipsa loquitur, there may be no direct evidence of what the defendant did; the person shows the accident is the kind that ordinarily happens only through negligence, and that the defendant controlled what caused it. The details change from state to state; this guide uses California as the worked example.

The starting point: ordinary negligence

Cornell’s Wex encyclopedia, a secondary educational source, defines negligence as “the failure to behave with the level of care that a reasonable person would have exercised under the same circumstances” (Wex: negligence). It lists the usual elements as a legal duty, a breach of that duty, harm, and two kinds of causation: cause-in-fact and proximate cause.

Breach is often the hardest element to prove, because a jury must decide what a reasonable person would have done. Both doctrines offer another route to that answer.

Negligence per se: borrowing a safety law

Wex describes negligence per se as “negligence in itself”: a defendant who violates a statute or regulation without excuse is treated as having breached the duty of care (Wex: negligence per se). Its most common use, according to Wex, is traffic violations. The plaintiff still has to show that the violation caused the injury.

In California, the rule is written into the Evidence Code. Section 669 says the failure to use due care is presumed if four things are shown (Cal. Evid. Code § 669):

• the person violated a statute, ordinance, or regulation of a public entity;
• the violation proximately caused death or injury to person or property;
• the harm came from an occurrence of the kind the law was designed to prevent; and
• the injured person was in the class of persons the law was adopted to protect.

The last two elements turn on the law’s purpose. California’s official jury instructions note that the first two elements are normally questions for the jury, while the trial court decides the last two as a matter of law (CACI No. 418). The same commentary quotes a California court calling negligence per se “not a separate cause of action” but an evidentiary presumption inside a negligence claim.

Excuses: when a violation is not the end of the story

In California, section 669 lets the presumption be rebutted. One route is proof that the violator did what a person of ordinary prudence, wanting to obey the law, might reasonably be expected to do in similar circumstances. A second route covers children who used the care typical of their age and maturity, but not when the activity is one normally done only by adults. Wex summarizes the Restatement approach, which excuses a violation in situations such as an unclear statute, a reasonable effort to comply, or cases where breaking the rule caused less harm than following it would have.

If the presumption is not established, California’s jury instruction tells jurors they “must still decide” whether the person was negligent under the other instructions.

Res ipsa loquitur: when the accident speaks for itself

Res ipsa loquitur is Latin for “the thing speaks for itself.” Wex describes it as a way to prove negligence through circumstantial evidence and lists three elements: the incident “does not generally happen without negligence,” it was caused by something solely in the defendant’s control, and the plaintiff did not contribute to the cause (Wex: res ipsa loquitur). Wex notes that the doctrine fails where the injury could result from the plaintiff’s own fault, giving some slip-and-fall accidents as an example.

California’s jury instruction uses the same three elements in plain words (CACI No. 417): the harm “ordinarily would not have happened unless someone was negligent,” it was caused by something only the defendant controlled, and the plaintiff’s voluntary actions did not cause or contribute to the events.

Inference or presumption? It depends on the state

States do not agree on how much weight res ipsa loquitur carries. In California, section 646 makes it “a presumption affecting the burden of producing evidence” (Cal. Evid. Code § 646). Once the defendant offers evidence that would support a finding of no negligence, the jury may draw an inference of negligence from the facts, but it may find negligence only if it believes that is more probable than not after weighing all the evidence. CACI No. 417 puts it this way: jurors “may, but are not required to,” find negligence.

Other states frame it differently. Wisconsin’s pattern civil jury instructions list res ipsa loquitur under the heading “Permissive Inferences” (Wis. JI-Civil 356). Negligence per se varies too: California’s section 669 sits in the Evidence Code article on presumptions affecting the burden of proof, a different category from section 646. A lawyer licensed in your state can explain which version applies.

Side by side

• What the plaintiff relies on. Negligence per se: a statute, ordinance or regulation the defendant violated. Res ipsa loquitur: the nature of the accident and the defendant’s control over its cause.
• What it replaces. Both help prove a breach of the duty of care. Neither removes the need to prove harm.
• Who it protects. Negligence per se applies only if the plaintiff is in the class the law protects and the harm is the kind it targets. Res ipsa loquitur has no such limit but requires that the plaintiff did not contribute to the event.
• How the defendant responds. In California, a negligence per se presumption is rebutted by a recognized excuse; a res ipsa presumption gives way once the defendant offers evidence supporting a finding of no negligence, leaving the jury to weigh everything.

Questions to sort out which doctrine might apply

• Did the other party break a specific safety law? Note the exact statute, ordinance or regulation.
• Was the law meant to prevent this kind of harm? And was the injured person in the group it protects?
• Is there direct evidence of what went wrong? If not, ask whether the accident ordinarily happens without carelessness.
• Who controlled the thing that caused the harm? Shared or uncertain control makes res ipsa loquitur harder to use.
• Which state’s law applies? Elements, excuses and the weight each doctrine carries differ by jurisdiction.

This guide explains general legal concepts and is legal information, not legal advice. California is used as an example; other states apply both doctrines differently. Consult a lawyer licensed in the relevant jurisdiction about a specific situation.